Gas sensors are employed in a variety of applications requiring qualitative and quantitative gaseous determinations. In the automotive industry, it is well known that the oxygen concentration in the automobile exhaust has a direct relationship to the engine air-to-fuel ratio. Oxygen gas sensors are employed within the automobile internal combustion control system to provide accurate exhaust gas oxygen concentration measurements for determination of optimum combustion conditions, maximization of efficient fuel usage, and management of exhaust emissions.
Generally, the electrochemical type of oxygen sensor employed in automotive applications utilizes a thimble shaped electrochemical galvanic cell to determine, or sense, the relative amounts of oxygen present in the exhaust stream, an example being U.S. Pat. No. 3,844,920 to Burgett et al. This type of oxygen sensor is generally known and used throughout the automotive industry, and comprises an ionically conductive solid electrolyte material, typically yttria stabilized zirconia, a porous electrode coating on the exterior exposed to the exhaust or measuring gas and a porous electrode coating on the interior exposed to a known concentration of reference gas. The gas concentration gradient across the solid electrolyte produces a galvanic potential which is related to the differential of the partial pressures of the gas at the two electrodes by the Nernst equation: E=AT 1n[P.sub.1 /P.sub.2 ], where E is the galvanic voltage, T is the absolute temperature of the gas, P.sub.1 /P.sub.2 is the ratio of the partial pressures of the reference gas at the two electrodes, and A=R/4F, where R is the universal gas constant and F is the Faraday constant. Thus, the oxygen sensor senses the oxygen concentration in the exhaust gas by measuring this galvanic output voltage.
As evidenced by the above recitation of the Nernst equation, the galvanic output voltage of the sensor is dependent on temperature. In addition, the solid electrolyte member within such a sensor must first be heated to an elevated temperature in order to obtain an appreciable output voltage in response to the difference in the oxygen concentrations between the reference and measuring electrodes. The induced galvanic potential between electrodes and the corresponding output voltage are not stable until the solid electrolyte has been heated to a given temperature. In general, the conventional oxygen sensors which do not have means for self-heating, rely on the combustion gases to heat the solid electrolyte member of the oxygen sensor to an operating temperature sufficient to effect galvanic stability. Effective sensor operation is therefore delayed until the combustion gases reach an appropriate elevated temperature so as to thereby heat the solid electrolyte within the sensor to an appropriate operational temperature.
If the oxygen sensor is placed too far downstream in the exhaust pipe of an engine, especially a highly efficient engine, the sensor may not be heated to a high enough temperature during engine warm up to meet sensor specifications. During these conditions, the internal combustion engine control system operates open loop, i.e., the control system does not sense the controlled parameter--the air-to-fuel ratio--even though programmed to control that parameter. It is known that a large percentage of the total emissions produced during a short period of operation are produced during this period of engine warm up. In some applications, emissions control during engine warm up would be improved with an oxygen sensor which had means for rapidly heating itself to a predetermined temperature, regardless of the temperature of the surrounding environment.
Further, it is known that the temperatures of the combustion gases from an internal combustion engine vary widely during operation, up to about a few hundred degrees Centigrade. Therefore, another advantage of a self-heating oxygen sensor is that it may be positioned anywhere in the automobile exhaust pipe, even at the cooler exit end, since the solid electrolyte of the sensor is not dependent on the heat of the combustion gases for heating itself. By locating the heated oxygen sensor at the cooler exit end of the exhaust pipe, it is significantly less degrading to the physical and chemical properties of the sensor than being disposed at the hot end of the exhaust pipe.
In summary, there is a strong motivation to provide an oxygen sensor capable of heating itself. Many heated oxygen sensors have been previously proposed in the art. These prior heated oxygen sensors generally comprise an elongated ceramic heater which positively heats the solid electrolyte body of the sensor. The heater element is typically inserted into an elongated cylindrical hole formed in the solid electrolyte body. An example of a prior heated oxygen sensor of this type is United Stated Ser. No. 110,353 to Ker et al, entitled "Heated Solid Electrolyte Oxygen Sensor", which is assigned to the same assignee of this patent application.
However, there are difficulties associated with the assembly of such an elongated heater element within the oxygen sensor. It is necessary that the heater element be easily and precisely positioned, while rigidly secured within the sensor element. Further, for automotive applications particularly, a heated oxygen sensor should be rugged, reliable, and readily manufacturable at a low cost. Therefore, it is also desirable that the heater components be readily adaptable to the current oxygen sensor design and manufacturing techniques.
Therefore, what is needed is a heated oxygen sensor wherein the heater element is readily located and rigidly secured within the oxygen sensor. In addition, such a heater for this type of oxygen sensor should be amenable to automotive mass production techniques by being relatively simple in design, but also a rugged, reliable sensor assembly. Lastly, it is preferable that the heater element be readily incorporated into conventional unheated oxygen sensors typified by the above mentioned U.S. Pat. No. 3,844,920 to Burgett et al.